This invention relates to a device for storing digital data. The preferred embodiment is described in connection with a system for establishing and maintaining one or more duplicate or "shadow" copies of stored data to thereby improve the availability of the stored data.
A typical digital computer system includes one or more mass storage subsystems for storing data (which may include program instructions) to be processed. In typical mass storage subsystems, the data is actually stored on disks. Disks are divided into a plurality of tracks, at selected radial distances from the center, and sectors, defining particular angular regions across each track, with each track and set of one or more sectors comprising a block, in which data is stored.
Since stored data may be unintentionally corrupted or destroyed, systems have been developed that create multiple copies of stored data, usually on separate storage devices, so that if the data on one of the devices or disks is damaged, it can be recovered from one or more of the remaining copies. Such multiple copies are known as the shadow set. In a shadow set, typically data that is stored in particular blocks on one member of the shadow set is the same as data stored in corresponding blocks on the other members of the shadow set. It is usually desirable to permit multiple host processors to simultaneously access (i.e., in parallel) the shadow set for read and write type requests ("I/O" requests).
It is sometimes necessary to "merge" two (or more) storage devices to reassemble a complete shadow set, where the devices were previously members of the same shadow set, but currently contain data that is valid, although possibly inconsistent. Data in a particular block is valid if it is not erroneous, that is, if it is correct, as determined by an error correction technique, or, if it is incorrect but correctable with use of the error correction technique. Shadow set members have data that is inconsistent if they have corresponding blocks whose data contents are different. For example, if one of the hosts malfunctions (e.g., fails), it may have had outstanding writes that completed to some shadow set members but not to others, resulting in data that is inconsistent. A merge operation ensures that the data stored on corresponding blocks of the shadow set members are consistent but does not determine the integrity (i.e., accuracy) of the data stored in the blocks which were inconsistent. The integrity of the data is verified by higher level techniques (e.g., by an applications program).